1. Field of the Invention
This invention relates to a method for rendering initiation of stretching easier in producing biaxially stretched film by enclosing gas inside a plastic tube and an apparatus used for this method.
2. Description of Prior Art
As one of methods for biaxially stretching plastic films, there have been known in general a method and apparatus therefor in which an unstretched tube (herein referred to as a raw film) is drawn out, passed through delivery nip rolls, a heating part, take-up nip rolls and after it is heated up to a suitable temperature, a pressurized gas is enclosed inside said raw film to effect inflation and at the same time, biaxial stretching is initiated by setting the roll speed of take-up nip rolls several times faster than that of delivery nip rolls. According to this method, every time when gas leakage at nip rolls or breakage due to accident occurs, stretching-initiating operations including operation for enclosing pressurized gas into a raw film must be carried out. Accordingly, it is desirable that the above-mentioned operations can be carried out easily.
Among such methods which have been known for initiation of stretching, there are the methods of Japanese Pat. publications No. 20876 of 1969 and No. 1316 of 1974, but both are not perfect in the point of handling.
According to the former method, connection of a special tube for enclosing pressurized gas with a raw film is carried out at the time of initiation of stretching, but this method requires quickness and skillfulness of operations, and is insufficient in the workability at the time of repetition required when initiation of stretching failed. According to the latter method, nip rolls having grooves are used in order to feed air into a tube. This method is effective for raw films having a width larger than a definite width, but in case of raw films having smaller width, the passage of gas at the part nipped with the above-mentioned grooved rolls is extemely bad and unstable and trouble is liable to occur at the time of initiation of stretching.
An object of the present invention is to provide a method in which the above-mentioned drawbacks of the conventional methods have been overcome and operation of initiation of stretching has been rendered easier.
Another object of the present invention is to provide an apparatus by which operation of initiation of stretching has been rendered easier without accompaniment of the drawbacks of the conventional apparatuses.
The above-mentioned objects and other advantages can be attained according to the method and apparatus of the present invention.